This application claims priority on German Application No. 19940952.8, filed Aug. 20, 1999.
The invention concerns a rate-adaptive cardiac pacemaker comprising sensor means for detecting at least one measurement parameter in the body of the patient, which is related to the circulation function, stimulation means for producing and outputting stimulation pulses to the heart of the patient, and rate control means which are connected at the input side to the sensor means and at the output side to the stimulation means, for controlling the rate of the stimulation pulses in dependence on the measurement parameter which is related to the circulation function.
Frequency- or rate-adaptive cardiac pacemakers are known in a wide range of different forms and have long been in clinical use. As they can react to changes in the physiological needs of the patient with changes in the stimulation frequency, it is possible to effectively take account of changing patient loading and strain states.
The design of a rate-adaptive pacemaker can be based on very different approaches in terms of control and regulation procedures. Basically, it is all the more promising in terms of success, the more extensively it takes advantage of the physiological interrelationships between loading and pulse rate in the patient with a sound heart.
Particularly advantageous from that point of view are sinus node-controlled units which directly sense the electrical sinus node or atrium activity. Those atrium-synchronous pacemakers are regularly used in connection with syndromes involving atrio-ventricular conduction interference with an intact sinus node function. In such cases the absolute value of the stimulation rate is predetermined at any time directly by the body-specific stimulation system under the guidance of the central nervous system (CNS).
Matters are different in regard to frequency-adaptive pacemakers which can be used in the therapy of syndromes involving disturbances in the sinus node function: Pacemakers of that kind use in various ways items of sensor information relating to the physiological needs of the patient and/or the physical activity of the patient. Those measurement values and the changes in the stimulation frequency, which are ascertained therefrom, are however basically relative in naturexe2x80x94see for example U.S. Pat. No. 3,593,718 which gives one of the first descriptions of a (respiration-controlled) rate-adaptive pacemaker. In other words, pacemakers of that kind do not involve information about the physiologically correct absolute value of the stimulation frequency. Nonetheless, the function thereof, on the basis of that relative stimulation frequency parameter, can be satisfactory in many situations of use as maladjustments of the absolute stimulation frequency are compensated by the body-specific regulation of the heart beat volume. Compensation in respect of rate maladjustments by the heart beat volume however represents an ongoing additional loading and strain on the patient and in addition naturally involves certain limits in regard to patients with reduced volume adaption capability.
For example U.S. Pat. No. 4,884,576 proposes patient-specific calibration of the characteristic curve which serves to determine the stimulation rate, between a lower and an upper limit rate, in which case specifically the long-term average value of the respiration signal is intended to serve to establish the configuration of the characteristic curve and optionally additional signals are intended to serve for displacement within the fixedly predetermined stimulation frequency band.
The object of the present invention is to provide a cardiac pacemaker of the general kind set forth, in which maladjustments of the absolute stimulation rate over the entire relevant adjustment range are avoided.
That object is attained by a cardiac pacemaker having the features recited in claim 1.
The invention embraces the concept of providing the pacemaker with means for obtaining sensor information about the physiologically correct absolute value of the stimulation frequency. As natural heart rate information is not available for the pacemaker types which are involved here, the invention further involves the notion of replacing same by sensor information about another available parameter which is regulated in terms of central nervous system. An advantageous starting point in this respect is the central-nervous coupling of respiration and circulation activity, in which in particular heart rate, heart beat volume and vasomotor system (here in particular the vessel resistance) are of significance as circulation-relevant parameters. As the pacemaker rate is intended to reproduce the physiological heart rate as accurately as possible, the invention in particular embraces the notion of obtaining a physiologically correct absolute value for the stimulation rate, by means of suitable processing of a respiration signal.
In the case of a healthy human body, close coupling occurs between respiration activity and base heart rate within given frequency relationships primarily in rest phases, especially while asleep at night. The frequency relationship which occurs can depend on whether the patient is active in a sport context. As a general trend for example for endurance athletes the heart/respiration frequency relationship is about 3, while for people who are less active in a sporting sense it is about 4.
Advantageously, by virtue of the ease of technical implementability, provided for the detection of respiration signals is an impedance sensor which is modified in regard to signal evaluation (impedance pneumograph) which has long been known as such and which is described for pacemakers for example in EP 0 151 689 B1 or EP 0 249 818 A1. Measurement of the respiration rate can be effected either alternatively or jointly by measurement of the intrathoracal and/or the intracardial impedance. If both intrathoracal and also intracardial impedance are measured, then those measurements can be utilized for mutual signal cleaning.
In consideration of the above-mentioned relationships, firstly (nighttime) rest phases are to be detected by the pacemaker. They can be recognized by virtue of the information from other sensors, for example by the detection of rapid eye movement phases. EP 0 502 918 B1 makes use of activity sensor data for detection of rest or sleep phases.
It is particularly desirable however for the respiration rate sensor which is provided to obtain the relevant measurement data also to be used for the detection of nighttime rest phases. Sleep phases can thereby generally be detected on the basis of a given level and in particular a given stability of the respiration rate.
The rate adaptation algorithm of the cardiac pacemaker can be retained independently of changes in the base stimulation rate. The rate ascertained on the basis of the algorithmxe2x80x94so-to-speak the dynamic componentxe2x80x94is then displaced towards higher or lower rates, in accordance with an offset which is ascertained from the respiration signalxe2x80x94the static component. It is then to be expected that the average stimulation frequency also changes to a similar degree to the base stimulation rate.
On the other hand there is the possibility of varying the rate adaptation algorithm in such a way that, upon changes in the base stimulation rate, the original maximum rate is still attained. For that purpose the gradient of the rate control characteristic curve must be raised or reduced in inverse proportion to changes in the base stimulation rate.
The invention can be used to particular advantage in relation to cardiac pacemakers which, for dynamic rate adaptation purposes, evaluate complex curve shapes such as the variation in respect of time of the intracardial impedance, the ventricular evoked response (VER) or the monophasic action potential (MAP). When evaluating such curves for the purposes of deriving rate control signals, the difficulty is frequently encountered that a reference curve configuration which is to be correlated to the base stimulation rate of the cardiac pacemaker has to be found for the resting patient. In regard to such cardiac pacemakers, the invention permits simultaneous detection of a reference curve configuration and the associated base stimulation rate.
A further development of the invention in regard to such cardiac pacemakers which evaluate complex curve shapes of cardiac activity signals provides that a medium stable load state is also detected on the basis of respiration activity (that is to say the rate thereof=respiration frequency and stroke capacity=respiration volume) in order to be able to record a reference curve for medium load states. More specifically as the coupling between respiration and heart rates disappears with increasing load, it is no longer possible to absolutely determine a heart rate at medium loads solely on the basis of the respiration rate and respiration rate and stroke capacity have to be detected for the purposes of detecting the load state. It will be noted that in that respect a further support location is also to be obtained in order to define the parameters of the rate adaptation algorithm and complete auto-calibration of the rate adaptation algorithm is made possible solely by means of a respiration sensor.